In recent years, concrete structures such as reinforced concrete columns and beams have received significant damage due to the large earthquakes that have occurred over many places. Investigations point out that the cause of the damage is due to the shear failure of concrete structures. Investigations identify that such damage is caused by the increasing of shear failure due to multiple alternating cyclic deformation during strong earthquakes. Thus, effective restraining of concrete structures is known to be important, in order to prevent shear failure of concrete structures and secure the stability and persistence for resilience characteristics against such repeated force.
A variety of types of reinforcement have been conventionally provided for existing reinforced concrete columns in order to increase the earthquake resistance of the reinforced concrete columns.
An example of such a reinforcing method is such that a pair of steel plates, a cross-sectional surface of which forms a U-shaped, are arranged around a reinforced concrete column, and end portions of the steel plates are butted to each other. The butted portions are welded to joint the steel plates with each other, and grout material is filled into a space portion formed in between the reinforced concrete column and the steel plates. According to this reinforcing method, the steel plates can be closely adhered to the reinforced concrete column via the grout material, thereby reinforcing the reinforced concrete column.
In this reinforcing method, since the end portions of the pair of steel plates are welded with each other, sparks produced during the welding operations may ignite flammable materials in the periphery to cause a fire. Moreover, it is necessary to place the end portions of the pair of steel plates accurately with each other, which requires accuracy in butting of end portions in the butted portions.
Concrete structures for forming a tunnel, which pierces a mountain and through which rail tracks and roads are laid, and concrete structures for forming stores and houses, which utilize the space underneath elevated railway tracks, are required to be reinforced for earthquake resistance, due to long-term deterioration.
When a reinforced concrete column is reinforced using a method for welding a steel plate including the above-mentioned reinforcing method, steel plates of 9 mm or more in thickness are required. Since such steel plates are heavy, heavy machinery will be essential for the transportation and construction of the steel plates. However, it is often the case that such heavy machinery cannot be brought into tunnels or underneath the elevated railway tracks. Thus, construction that can be performed with only manpower is desired.
A reinforcing method for binding a reinforced concrete column without welding steel plates is also proposed. For example, Japanese Laid-Open Publication No. 9-177334 (Patent Literature 1) proposes a technique of overlapping end portions of steel plates 40 enclosing a reinforced concrete column 1, connecting the end portions with bolts and nuts 41, and filling grout material into a space portion 42 in between the reinforced concrete column 1 and the steel plates 40, as shown in FIG. 15.
Japanese Laid-Open Publication No. 10-220030 (Patent Literature 2) proposes a technique of forming a teeth-shaped engaging portion 44 at both end portions of an steel plate 43, overlapping and engaging the engaging portions 44 and 44 of the adjacent steel plates 43 and 43 to fasten the steel plates 43 and 43, and filling grout material into a space portion 42 in between the reinforced concrete column 1 and the steel plates 43, as shown in FIG. 16.
According to these reinforcing methods for reinforced concrete columns, there is no need to worry about fire hazard due to welding, and accuracy is not required for the butting of the end portions of the butted portions.
Furthermore, in order to reinforce the column 1 in a sufficient manner, it is necessary to secure a space portion of an appropriate size in between the steel plates and the column 1 and fill the space portion with a predetermined amount of grout material. If the amount of the grout material to be filled is less than necessary, then the reinforcement will not be sufficient. On the other hand, if the amount of the grout material to be filled is more than necessary, then the cost for the construction will increase.
However, with the reinforcing methods described in Patent Literatures 1 and 2, the size of the space portion in between the steel plates and the reinforced concrete column 1 is determined in accordance with the size of the steel plates and column. Thus, it is not possible to adjust the size of the space portion appropriately in accordance with the size or the like of the column 1. Furthermore, since the subject reinforcing methods reinforce columns using only the steel plates and grout material, the reinforced, reinforced concrete column does not have sufficient toughness against repeated deformation during strong shakes. Thus, the shear capacity is low.
Thus, Japanese Laid-Open Publication No. 2005-23745 (Patent Literature 3) proposes a reinforcing method, where four steel plates 45, cross-section of which is formed in an L-shape, are arranged around the corner portions of a reinforced concrete column 1 as shown in FIG. 17. End portions of the adjacent steel plates 45 and 45 are overlapped with each other, and then a belt-shaped fiber sheet 46 is wound around the outer periphery of the steel plates 45 to bind the four steel plates 45. Grout material is filled into a space portion 42 formed in between the four steel plates 45 and the reinforced concrete column 1.
According to this reinforcing method, the steel plates 45 can be slid in accordance with the size of the outer shape of the reinforced concrete column 1, which means that the size of the space portion 42 in between the steel plates 45 and the reinforced concrete column 1 is changeable. Furthermore, if a shear force works on the reinforced concrete column 1, the steel plates 45 will slide with each other so that the force will be transmitted to the belt-shaped fiber sheet 46. Thus, the toughness of the reinforced concrete column 1 will be increased and the earthquake resistance will be improved for the reinforced concrete column 1.
However, if the shear force working on the reinforced concrete column 1 is too great, all the shear force may work on the belt-shaped fiber sheet 46 and the belt-shaped fiber sheet 46 may be cut off. Although the belt-shaped fiber sheet 46 is normally cut for the first time when in contact with sharp objects such as a blade, it will be naturally cut off if the shear force working on the belt-shaped fiber sheet 46 is too great and rapidly and exceeds the limit of the tension of the belt-shaped fiber sheet 46. If the belt-shaped fiber sheet 46 is cut off, the reinforced concrete column 1 will be completely destroyed.
The techniques disclosed in these prior art documents are all on the premise that there is a space around the column to be reinforced, and a wall surface is not in contact with the column and not in connection with each other. Accordingly, the abovementioned technique cannot be applied for the reinforcing measures of, for example, concrete columns constituting a tunnel, in a formation where the wall surface is in connection with the column.
A reinforcing method of a reinforced concrete column for a formation where a wall surface is in contact with a column is described in Japanese Laid-Open Publication No. 2012-7418 (Patent Literature 4).
As shown in FIG. 18, this reinforcing method is such that a pair of steel plates 52 and 52 are slidably arranged against an outer wall surface of a reinforced concrete column 1, the pair of the steel plates 52 and 52 are bound by a reinforcing sheet 47, and grout material 57 is filled into a space portion formed in between the pair of steel plates 52 and 52 and the reinforced concrete column 1. Note that the numeral 53 in the figure denotes a wall.
This reinforcing method, however, is rather a method for increasing the strength and bending strength of the reinforced concrete column 1, and it will not increase the toughness of the reinforced concrete column 1. Thus this reinforcing method has a great defect of making the strength between the entire structure and the column imbalanced and the earthquake resistance will not be improved.